1. Technical Field
The disclosed system and method generally relates to the field of wireless communications. More particularly, the disclosed system and method relates to a system and method for compensating for I-Q mismatch in a low intermediate frequency (IF) or zero IF receiver.
2. Description of Related Art
Radio receivers, such as a heterodyne receiver, have long been used for radio communication. With a heterodyne architecture, a radio frequency signal is detected by a tuner, or other radio frequency device coupled to an antenna. An internal oscillator, called a local oscillator, is supplied to a mixer along with the radio frequency signal. The mixer produces output signals at both the sum and difference between the radio frequency and the local oscillator. The output of this stage is usually designated as an intermediate frequency (IF). Because the IF is still relatively high frequency, conventional filtering techniques may be used to eliminate one set of output signals from the mixer (i.e., either the sum or the difference signals).
Heterodyne techniques have been used in many kinds of receivers. For example, wireless communication devices, such as cellular telephones, often use heterodyne architecture. However, this architecture requires additional circuitry, power consumption and additional expense to build the device. Thus, new system architectures are arising in which the IF circuitry is eliminated. Receivers employing this architecture are sometimes referred to as zero IF receivers. In this application, the local oscillator mixes the radio frequency signal directly to baseband frequencies. In a similar architecture, designated as a low IF architecture, the local oscillator mixes the RF signal down to an IF. However, the IF is a very low frequency and thus does not permit the conventional filtering to remove the undesirable image band interference as is common in heterodyne architectures, as described above.
The zero IF and low IF receivers have virtually identical front end circuitry. An example of this system architecture is illustrated in FIG. 1 in which a quadrature receiver employs zero IF or low IF architecture. As illustrated in FIG. 1, a conventional system 10 includes an antenna 12 coupled to a radio frequency (RF) stage 14. The RF stage 14 may include amplifiers, filters, tuning elements, and the like. Details of the RF stage 14 are known to those skilled in the art and need not be described herein. The RF stage 14 operates in conjunction with the antenna 12 to detect a modulated RF signal and generates an electrical signal corresponding thereto.
The conventional system 10 also includes an RF splitter 16, which generates two identical copies of the signal from the output of the RF stage 14. The RF splitter 16 may be an electrical circuit or, in its simplest implementation, it may simply be a wire connection. In some implementations, the RF splitter 16 may be implemented as part of the RF stage 14.
The two identical signals are provided to RF inputs of a mixer 20 and a mixer 22. The mixers 20 and 22 each include a local oscillator input. The mixer 20 is provided with the local oscillator signal, designated as an xe2x80x9cIxe2x80x9d local oscillator. The mixer 22 is provided with a local oscillator signal, designated as a xe2x80x9cQxe2x80x9d local oscillator. The local oscillator signals I and Q are identical in frequency, but have a 90xc2x0 phase shift with respect to each other. Techniques for producing these quadrature signals are known in the art and need not be described in detail herein. The output of the mixers 20 and 22 are provided to low-pass filters 24 and 26, respectively. In an exemplary embodiment, the filters 24 and 26 are low-pass filters. The resultant signal generated by the filter 24 is a baseband (or near baseband) signal I(t). Similarly, the resultant signal generated by the filter 26 is a baseband (or near baseband) signal Q(t).
In ideal circumstances, the quadrature signals provided by the I local oscillator and the Q local oscillator are separated by precisely 90xc2x0. The resulting I and Q outputs would, ideally, have equal amplitudes. Further, an ideal system would have precisely matched mixers 20 and 22 and matched filters 24 and 26. Under these ideal circumstances, the output I(t) and Q(t) are truly orthogonal. That is, there is no projection of the I(t) signal into the Q(t) signal and vice-versa.
Unfortunately, such ideal circuits do not exist. Even with close matching of the mixers 20 and 22 and the filters 24 and 26, some phase and/or gain errors will result. This undesirable circuit mismatch in the I and Q circuits results in output signals I(t) and Q(t) that are not truly orthogonal. That is, the I(t) signal may project onto the Q(t) signal and vice-versa. The results of this circuit mismatch are illustrated in FIGS. 2A and 2B. The results of circuit mismatch affect both I(t) and Q(t); thus, we will consider the complex spectrum of the quadrature signals in the discussion with respect to FIGS. 2A and 2B.
FIG. 2A is an RF spectrum. Those skilled in the art will recognize that, for the sake of convenience, the RF spectrum is not drawn to scale. The RF spectrum includes a line 30 representing the I local oscillator signal. The desirable signal is indicated by a portion 32 of the spectrum. FIG. 2A also illustrates what are designated as xe2x80x9cjammerxe2x80x9d signals that are present due to adjacent channels or alternate channels. The adjacent channel, separated from the carrier frequency of the desired signal by 30 kilohertz (kHz), is indicated by a portion 34 of the spectrum labeled as the J_30 signal.
Telecommunications standard IS-98B, entitled xe2x80x9cRF Performance for Dual-Mode Mobile Telephones,xe2x80x9d specifies the measurement of certain interference signals using a jammer signal that is separated from the desired carrier frequency by 60 kHz. A portion 36 of the spectrum indicates the presence of the J-60 jammer signal. In addition, FIG. 2A illustrates a portion 38 of the spectrum resulting from a jammer signal J_120, which is separated from the carrier frequency of the desired signal 32 by 120 kHz.
Those skilled in the art will appreciate that the spectrum is symmetrical about the DC axis (0 Hz). Thus, the spectrum 32 of the desired signal has a mirror image spectrum 32xe2x80x2, which is centered at the minus carrier frequency. Similarly, the spectrum 34, 36, and 38 each have mirror image spectra 34xe2x80x2, 36xe2x80x2, and 38xe2x80x2, respectively.
FIG. 2A also illustrates a line 40 indicating a portion of the spectrum resulting from a local oscillator signal due to mismatch between the I and Q portions of the circuit illustrated in the example circuit of FIG. 1. The mixers 20 and 22 multiply the signals in the RF spectrum by the value of the local oscillator 30. The result of processing the portions 32-38 and 32xe2x80x2-38xe2x80x2 by the local oscillator 30 is effectively a shift in frequency of all components in the spectrum of FIG. 2A. Following processing by the mixers (e.g., the mixer 20) and the filters (e.g., the filter 24), the I circuit of FIG. 1 produces the baseband spectrum illustrated in FIG. 2B. The spectral portions 32-38 and 32xe2x80x2-38xe2x80x2 have effectively been shifted to the right by the frequency of the local oscillator. As a result, the portion 32xe2x80x2 of the spectrum, which represents the desired signal, is now centered at 15 kHz. Similarly, the portions 34xe2x80x2, 36xe2x80x2, and 38xe2x80x2 of the spectrum have been frequency shifted and are now centered at xe2x88x9215 kHz, xe2x88x9245 kHz, and xe2x88x92105 kHz, respectively. At the same time, the portions 32-38 of the spectrum (see FIG. 2A) have been shifted to a much higher frequency level and are not illustrated in FIG. 2B. Those portions of the spectrum are undesirable and are readily removed using conventional techniques.
The mismatch local oscillator 40 also interacts with the portions 32-38 and 32xe2x80x2-38xe2x80x2 of the RF spectrum illustrated in FIG. 2A. While the positive frequency value of the local oscillator 30 effectively shifts the RF spectrum in the positive frequency direction, the negative frequency value of the mismatched local oscillator 40 effectively shifts the RF spectrum in the negative frequency direction. As a result, the portions 32xe2x80x2-38xe2x80x2 are shifted in the negative frequency direction such that they cause no interference with the desired signal centered at 15 kHz in FIG. 2B. However, the portions 32-38 of the spectrum in FIG. 2A are shifted to the left such that the original portion 32 in FIG. 2A is now centered at xe2x88x9215 kHz and is identified in FIG. 2B as a portion 32i to indicate that the portion 32i is a signal image resulting from the undesirable presence of the mismatch local oscillator 40. Similarly, the portions 34-38 of the spectrum in FIG. 2A are shifted in the negative frequency direction to produce spectral portions 34i-38i illustrated in FIG. 2B. It should be noted that the portion 34i is the J_30 image spectrum, which effectively creates sidebands in the baseband signal directly coincides with the desired signal spectrum centered at 15 kHz. In addition, portion 36i, representing the J_60 image spectra, is centered at 45 kHz and may also cause significant interference with the desired signal.
The undesirable sidebands 32i-38i may be characterized as xe2x80x9cresidual sidebandsxe2x80x9d because they result from the residual effects of the mismatch local oscillator 40. Careful matching of the circuit components for the mixers 20 and 22 and the filters 24 and 26 may reduce the residual sidebands and thus the undesirable image spectra. However, circuit matching cannot completely eliminate the mismatch local oscillator signal. Therefore, it can be appreciated that there is a significant need for a technique to compensate for I-Q mismatch in a zero IF or low IF system architecture. The present invention provides this and other advantages as will be apparent from the following detailed description and accompanying figures.
The present invention is embodied in an apparatus for the compensation of I-Q mismatch in a low IF or zero IF receiver and comprises first and second mixers having respective radio frequency (RF) inputs, local oscillator inputs, and mixer outputs. The RF inputs of the mixers are configured to receive modulated RF signals, the local oscillator of the first mixer is configured to receive an I local oscillator signal while the local oscillator input of the second mixer is configured to receive a Q local oscillator signal. The I and Q oscillator signals have substantially identical frequencies. The apparatus further comprises first and second filters coupled to the mixer outputs of the first and second mixers, respectively, to filter output signals from the mixer outputs and thereby generate I and Q output signals, respectively. Circuit differences in the first and second mixers and/or first and second filters result in gain and/or phase errors that result in mismatch in the I and Q output signals. The apparatus comprises a correction circuit to automatically apply a correction factor to at least one of the I and Q output signals to correct the gain and/or phase error by applying a multiplication factor to the at least one of the I and Q output signals to thereby generate a corrected signal.
In an exemplary embodiment, the correction circuit applies multiplication factors to both the I and Q output signals to thereby generate a corrected I output signal and a corrected Q output signal. The compensation circuit may be an analog circuit or a digital circuit. In one embodiment, the receiver is a portion of a wireless communication device and the system further comprises a storage area to store data indicative of the correction factor.
In an exemplary embodiment, a test signal generation circuit generates a fixed frequency signal as inputs to the first and second mixers to permit the test measurement of gain and/or phase errors. In this embodiment, the correction factor applied by the correction circuit is based on the test measurement. The correction factors may be stored in a storage area within the wireless communication device with the data in the storage area indicative of the correction factor.